POSITION OF FAULTS. 
267 
of divisions of the latter represents the intensity of the 
current. 
A curved magnet attached to an upright copper rod that 
rises above the apparatus, and can be placed at any height on 
this rod, gives the means of counterbalancing the terrestrial 
magnetism so as to augment the sensitiveness of the apparatus 
as required. The upright rod is turned round by an endless 
screw engaging a wheel attached to the rod, so that the spot of 
light can be exactly adjusted to the zero of the graduated 
scale. 
This apparatus is not only very accurate, but it is so easily 
and conveniently used in the tests to which the insulating 
materials of cables and its adjuncts are subjected, that its 
employment is now nearly universal. 
Let us suppose that a conductor, otherwise well insulated, 
has a fault at b (fig. 188) at a distance from the station a. 
If the contact of b with earth is perfect and has no sensible 
resistance, it will be sufficient in order to find the distance 
A b to measure the resistance of the length A b, and divide 
this resistance by that of the unit of length. This measure¬ 
ment may be effected by Wheatstone’s bridge as shown in 
the figure. D a and D F are the two branches of the balance, 
F e is a box containing variable and adjustable resistances (the 
box as actually used is exhibited in fig. 189) and z c is the 
battery which is thrown into action by the key p. If a d 
is one-tenth of d f, and the pegs taken from the box between 
F and E represent 1,500 when the galvanometer is.brought to 
zero, then abe has a resistance of 150 units ; and if the line 
has a resistance of five units per mile, the fault b is thirty 
miles from A. When this test is being made, the other end 
c of the line should be insulated. It is easy to find whether 
the resistance b e is nil, by repeating the experiment at c. If 
in this second test we obtain a distance b c, which, added to 
A B gives the total length, A c, the part b e can have no resis¬ 
tance. If, on the other hand, the sum of the resistances be 
greater than that due to a c, the excess is caused by the 
resistance of the fault. As we measure in one case A b -j- b e, 
and in the other bc|be, the sum of these exceeds the resis¬ 
tance of A c by double the resistance of the fault. Suppose 
the line is eighty miles long, the resistance A 0 will be 400 
